ODST - Iron

Prometheans. Though to be fair, they were vastly improved between H4 and H5. Regardless, in both iterations the Prometheans were bullet sponges. The Knights were too OP in H4 with ridiculous extra abilities like teleportation that allowed them to escape death or deliver death instantaneously. The Watchers were also OP because they could block grenades, provide cover, and resurrect allies. Not to mention, it took way too many shots to kill them and they were so small that you had to be precise with each shot in quick succession. The Crawlers weren't OP, but there were so many of them that they ate up your bullets. And since H4 had such scarce ammo, each bullet mattered.

The Flood on the other hand didn't have ridiculous abilities like Knights and didn't have the ability to block grenades or raise shields (though they did resurrect themselves if you didn't kill the Infection Forms) like Watchers. You could take them down with any number of weapons in any number of ways, which made game play more interesting imo.

Marine - Onyx

Marine - Bronze

Prometheans suffer from the same fundamental problem that the Brutes had in Halo 2 (i.e. they are just not that interesting to fight, and can be downright annoying at times). Therefore, I have to go with the Prometheans.

Marine - Gold

Prometheans are annoying bullet sponges, but at least when they go down they STAY DOWN. I have gotten so fed up with killing a combat form and even after sweeping the area for Infection forms, three Combat forms will still just get up and come at you. Plus their melee is bull crap.

Marine - Onyx

Recruit - Onyx

Prometheans are boring dull robots with orange juice coming out of them. Meanwhile the Flood is a manifestation of infection and plague that has been terrorizing worlds and spreading throughout the galaxy for thousand generations. A true force to be reckoned with. It's an obvious pick really.

Marine - Bronze

Prometheans are the worst enemy the Halo series has ever seen. Halo 4 was a bore for me personally. Shoot robot, robot dies, flying robot brings back robot. Robot mad, robot stab, retry. There was very little personality there. The flood was a hivemind of death, much more interesting